1. Field of the Invention
The present invention relates to an imaging apparatus which processes an image signal obtained by photoelectric conversion, and its control method.
2. Description of the Related Art
In recent years, a complementary metal-oxide semiconductor (CMOS) sensor has drawn much attention since it has low electric power consumption, shows a high signal-to-noise ratio (SN ratio) equivalent to a charge-coupled device (CCD), and its signal processing circuit can be manufactured by the same semiconductor process as a photoelectric conversion unit.
FIG. 9 is a diagram illustrating a readout path for a signal of one pixel and a configuration for processing a readout signal in the conventional CMOS sensor containing a plurality of pixels. When exposure of the CMOS sensor starts, first a switch SW1 is turned on and an electric charge accumulated in an area mainly comprised of a floating diffusion unit (FD) 502 is reset in the input portion of an amplifier 510. Then, after the SW1 is turned off, the electric charge of the FD 502 is read out into a capacitance CN (noise signal). Next, a switch SW2 is turned on to transfer the electric charge accumulated in a photodiode (PD) 501 which is obtained by photoelectric conversion, to the FD 502 and read out the charge into a capacitance CS (image signal). The image signal held in the capacitance CS and the noise signal held in the capacitance CN is differentiated by a correlated double sampling (CDS) circuit 504. Thus, an image signal from which a noise component is removed can be output. The image signal is further converted into a digital signal by an analog-to-digital (A/D) converter 505 and subjected to signal processing by a signal processing circuit 506.
In a CMOS sensor having the above-described configuration, it has been known that a phenomenon referred to as high-brightness darkening occurs. The high-brightness darkening is the phenomenon in which when a significantly large amount of light enters, an output signal abruptly disappears and an area irradiated with the light appears black as if the light does not enter.
It is considered that this high-brightness darkening is caused by electric charge which cannot be held by the PD 501 and flows into the FD 502 when a significantly large amount of light is incident on the pixel PD 501. Thus, if electric charge flows into the FD 502, a readout noise signal rapidly becomes large, so that the difference (output signal) between an image signal and a noise signal is reduced.
In Japanese Patent Application Laid-Open No. 2000-287131, in order to alleviate the high-brightness darkening, a rapid rise of the noise signal is detected by comparing the noise signal and a threshold value. Thus, the noise signal can be clipped to a fixed value.
However, although the noise signal is clipped, if an error occurs in the threshold value used for determining the darkening, or in the level of the clipped signal, the darkening may remain. This point will be described below.
FIGS. 10A to 10E illustrate the transition of an image signal obtained by the CMOS sensor in one line scanning when a high-brightness object such as the sun is shot at each point in the path of the CMOS sensor illustrated in FIG. 9.
FIG. 10 is a schematic diagram illustrating the amount of electric charge accumulated on each pixel of the CMOS sensor when the high-brightness object like the sun is shot. FIG. 10 B is a diagram illustrating one line of a noise signal N and an image signal S at the point P1 in FIG. 9. The vertical line indicates the level of signals and the horizontal axis indicates the position of pixels in a horizontal direction. In the image signal S, the amount of electric charge (incident level) obtained by photoelectric conversion in the PD 501 exceeds a saturation level. Thus, the saturation level will be continued. When the incident level reaches a supersaturation level which further exceeds the saturation level, electric charge leaks from the PD 501 to the FD 502. Thus, a value of the noise signal N becomes larger than the surrounding values. If the noise signal N is input to the CDS circuit 504 as it is and differential processing (S-N) between the noise signal N and the image signal S is carried out, the output level of the pixel exceeding the supersaturation level is significantly reduced compared with the surrounding values.
On the other hand, if a clipping circuit 503 is provided to clip the noise level to a threshold value when the noise signal N exceeds the threshold value (clipping level) (FIG. 10C), a decrease of level in a supersaturation area can be alleviated (FIG. 10D).
If the threshold value is set low, the darkening can be eliminated, however, it becomes highly possible that even a normal noise component can be clipped, which leads to deterioration of an image quality. Conversely, if the threshold value is set high, there is a possibility that the darkening may not be suppressed. FIGS. 10C and 10D illustrate input and output of the CDS circuit 504 when the darkening is not completely eliminated by the setting of the threshold value at points P2 and P3 respectively. In order to eliminate the unevenness of the saturation level for each pixel, also the A/D converter 505 clips the signal to an upper limit of a predetermined level (FIG. 10E). Here, if a range to be clipped is also reduced, the impact of the darkening can be eliminated. However, this causes degradation of an image quality. As a result, with respect to the output of the A/D converter 505, a pixel area which is at a lower level than the saturation level may be generated inside the pixel area of the saturation level. For example, when the sun is shot, the level of the center of the sun may be lower than the periphery of the sun.
Further, when the level of a noise signal fluctuates depending on a change in environment such as electric voltage and temperature, if only one clipping level is set and always applied to the signal, deterioration of an S/N ratio cannot be prevented.